The Astros announced Saturday afternoon they will place left-hander Wandy Rodriguez on the 15-day disabled list Monday with effusion (fluid) in his left elbow joint and will purchase the contract of Lyles, a right-handed pitcher, from Triple-A Oklahoma City. Rodriguez's placement on the DL will be retroactive to May 23.
Lyles, 20, the Astros' Minor League Pitcher of the Year last year, will make his Major League debut on Tuesday when he gets the start against the Cubs. He's 3-3 with a 3.20 ERA in 10 starts this year for the RedHawks. Lyles is also scheduled to start June 5 in San Diego. He'll be the fourth-youngest pitcher to start a game for the Astros.
"As a little kid, I always wanted to play in the Majors," Lyles said. "I never thought I'd pitch in the big leagues at 20 years old, but that will come Tuesday."
Rodriguez, meanwhile, underwent an MRI on Saturday morning because of discomfort in his elbow, and it showed no structural issues. He was scheduled to start Saturday's game against Arizona, but was scratched on Friday. Rodriguez is 3-3 with a 3.41 ERA in 10 starts this season, and is 3-1 with a 2.16 ERA in his last seven starts.
Astros general manager Ed Wade said he anticipates Rodriguez will miss only two starts. Rodriguez would be eligible to come off the disabled list on June 7.
"When he threw his side [bullpen session] the other day, the fastball and the changeup, he had no problems," Wade said. "He said he just didn't feel comfortable with the breaking ball. Dr. [Tom] Mehlhoff thought, just as a precaution, it made sense to get an MRI done. The MRI showed no structural issues at all. There was a little bit of swelling in the elbow, and the feeling is that he definitely wouldn't make his start Tuesday, which is the day we'll need somebody."
Lyles was scheduled to pitch Monday for Oklahoma City.
"It wasn't about, 'Is Jordan ready or not?'" Wade said. "We won't know if he's ready or not until he gets up here and gets a chance, and even if he comes up here and struggles in two starts, we're still not going to know if he's ready or not. We're not going to judge him in two starts."
In Lyles' last seven starts, he has been one of the top starting pitchers in Triple-A, posting a 3-1 record with a 2.00 ERA. In that span, he has allowed two runs or less in six starts, and one run or less in four starts.
"He's been pitching well," Mills said. "His last outing [Wednesday] was six shutout innings, and he only threw 77 pitches. He's put some pretty good outings together. Everyone's excited with the way he threw in Spring Training for us, and we're exited for him. This will be a good time to see him pitch."
Lyles, the 38th overall pick in the 2008 First-Year Player Draft, dazzled the Astros in Spring Training, posting a 1.98 ERA in five appearances while competing for the fifth-starter's job -- a spot that eventually went to Nelson Figueroa.
To make room for Lyles on the 40-man roster, catcher Jason Castro will be transferred to the 60-day disabled list.
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.